Being a Rails developer brings up the need to work with different Ruby versions, whether it is by a project requirement or to test new features coming out the oven... and the same applies to Rails.
With Rails being in a phase where version 3 is in development, you may (and should) want to take a look to its new features, but surely you don't want to touch your perfectly working Rails 2.3.x installation.
Also, you may be working with Ruby 1.8.x and need to test if your current project works fine with Ruby 1.9.x, so... is there something you can do to have several Ruby mixed with several Rails versions? Chances are you already have heard about Ruby Version Manager (RVM), the amazing tool that will be your best friend when you're dealing with this version gap.
Installing RVM
We are going to stick to the most straight-forward method, installing RVM as a user (there's also the option to do it as root and system wide) and from the Github repository. You already work with git, right? If not, check out this post.
bash < <( curl http://rvm.beginrescueend.com/releases/rvm-install-head )
This is the best option to install RVM in your machine and will work in Macintosh and *nix environments, can't promise anything about Windows, though.
There are other two options: from the latest source tarball and as a gem, but the guys from BeginRescueEnd recommend to stick to the described method.
After RVM is installed in your system, you need to add this line to your ~/.bashrc and ~/.bash_profile
[[ -s $HOME/.rvm/scripts/rvm ]] && source $HOME/.rvm/scripts/rvm
Now what?
Well, you should be ready to start playing around with any Ruby version you want to work with; but before that, it is a good practice to check the Ruby versions that RVM is aware of
rvm list known
To install a specific Ruby version, you just have to:
rvm install <ruby version>
When you already have several 'Rubies', you can make any of them the default version:
rvm --default <ruby version>
If you ever want (or need) to go back to your previously Ruby/Rails installed version, here's the trick:
rvm system
What about Rails???
The real awesomeness start when you discover that you can install as many Rails versions as you want; and the 'bestest' part is the ability to mix them up with your installed 'Rubies'!!!
Let's take one step at a time.
Gemsets
RVM provides a feature/concept called 'gemset'; which will allows us to install gems to test them with any of the Ruby versions you have installed.
To create a gemset you have to run:
rvm gemset create <name for your gemset>
I like naming my gemsets after the project I will use the Rails version / Gems version; most of the developers will prefer using a naming convention like 'Rails235', 'Rails300beta4', 'Rails3rc' or anything like that.
Now, to mix any of your Ruby version with any of your Rails versions, you'll use:
rvm use <ruby version>@<gemset name> (e.g.) rvm use 1.8.7@rails_300_RC
you can omit the word 'use' and it will work too
Now, install the Rails version you want to try within this gemset:
gem install rails -v 2.3.3
Forget about sudo to install gems
This is very important, since you need to install the gems to your current gemset folder, so avoid using sudo or else forget about the magic
Checking versions
This is pretty straight forward, but make sure to check your Ruby/Rails version each time you switch:
ruby -v && rails -v
What else is important?
Check on the documentation to become an expert using RVM; you can remove Ruby installations, delete-import-export gemsets and a whole bunch of features that will make your life easier (at least working with RoR versions)
Also, the BeginRescueEnd guys have a section for each OS; stop by and take a look.
Current Ruby selection in your prompt line
There is one interesting post in RafLabs about the subject and a very simple implementation by typing:
PS1="[\h:$(~/.rvm/bin/rvm-prompt)]\W \u$ "
This will allow your prompt to look a little bit like this:
[localhost:ruby-1.8.7-p299@r3rc] home~
or
[dab-mac:ruby-1.9.1-p378]~ home$